1. Field of the Invention
The present invention generally relates to cladding pumped fiber lasers and, more particularly, to an improved ytterbium (Yb)-doped cladding pumped fiber laser that operates at around 970 to 980 nm which is the absorption band of erbium doped fiber amplifiers. It also relates particularly to an improved method of wavelength discrimination to remove parasitic lasing that occurs at wavelengths longer than the desired wavelength in a cladding pumped fiber laser.
2. Description of Related Art
The continuing interest in fiber lasers over bulk lasers stems from the ability to achieve very long gain media that have negligible losses and a good overlap with pump beams. Fiber lasers also have lower pump thresholds. Another advantage of a fiber laser is that they operate at the lowest transverse mode that can be easily and efficiently injected into other fibers.
In fiber lasers, one goal has been to provide flexibility in operating wavelength, cavity length, and application. With more flexibility, there can be more useful applications of the laser. But in seeking maximum flexibility in those areas, a need to achieve wavelength discrimination arises. In the past, bulk elements such as prisms, gratings, and lenses inside the laser cavity have been used for wavelength discrimination. However, those elements not only use valuable space due to their bulk, they also create energy losses. To minimize losses, fiber gratings have been used as fiber mirrors to provide narrow linewidth operation at a given wavelength. Conventionally, reflective Bragg gratings have been photolithographically written into the fiber core to achieve lasing at a specific wavelength while suppressing another wavelength. But that technique provides less than adequate discrimination as the gain at unwanted wavelengths increases.
As a further example of an attempt to achieve wavelength discrimination, Vengsarkar et al., "Long-Period Fiber Gratings as Band-Rejection Filters, Journal of Lightwave Technology, Vol. 14, No. 1, pg 58 (1996) describe in-fiber, long-period fiber gratings that act as low-loss, band-rejection filters. However, Vengsarkar et al. teach that a single fiber grating be placed outside of a laser cavity after the light has been launched into a fiber. But doing so does not adequately address the problem of recurring, unwanted wavelength inside a laser cavity, which thereby results in lowering system efficiency at the desired wavelength or, ultimately, suppressing it completely.
Another desirable goal of fiber laser technology has been to achieve high power, multi-Watt output from the laser. Of course, with a greater range of laser output, there is a greater range of applications. But, in the past, it has been found that a cladding-pumped fiber laser geometry with multi-mode laser diodes as a pump source is the only practical means of achieving multi-Watt output. On the other hand, it is known that, in comparison to a non-cladding pumped geometry, a cladding pumped geometry increases the absorption length and so must the cavity length increase. Thus, if a cladding pumped geometry must be used, additional constraints are imposed in the construction of the laser.
Whether for cladding pumped or otherwise, it has been common practice in fiber laser construction to use rare earth elements as dopants. Any particular dopant will have a particular energy level transition scheme. When subjected to excitation, the dopant provides the lasing action which derives from its particular energy level transition scheme. Some of the common dopants are ytterbium (Yb), erbium (Er), praseodymium (Pr), and neodymium (Nd). The fiber materials have often been silica glass, germanosilicate or zirconium fluoride glass (ZBLAN).
The use of Yb for doping continues to gain interest over other dopants due to its high efficiency based on its simple energy structure. It has a ground manifold with four Stark levels and an excited manifold with three Stark levels, with the two manifolds being separated by about 10,000 cm.sup.-1. The relatively large separation of other energy levels from the two lowest manifolds eliminates excited state absorption (ESA) at either pump or laser wavelengths. The 10,000 cm.sup.-1 energy gap between the ground and excited manifolds also precludes nonradiative decay via multiphonon emission from the excited manifold, as well as precluding concentration quenching.
Yb-doped fibers exhibit broad absorption and emission bands--about 800 nm to 1064 nm for absorption and about 970 nm to 1200 nm for emission. There is a spectral range from about 970 to 1064 nm which comprises an overlapping area of absorption and emission. With a broad absorption band for Yb-doped fiber lasers, there can be a broad choice of pump wavelengths. Accordingly, Yb-doped fiber lasers provide an efficient and convenient means of wavelength conversion from different pump lasers, such as AlGaAs and InGaAs diodes, Ti:sapphire lasers, Nd:YAG lasers and Nd:YLF lasers.
Operation of Yb-doped fiber lasers has been achieved on a three-level scheme at around 970-980 nm and on a four-level scheme at around 1030-1200 nm. However, operation on a three-level scheme is considerably more challenging, since high pump intensities are required to bleach initial absorption. Also, ytterbium has often been used as a sensitizer ion to absorb pump power over a wide range and then transfer the excitation to an acceptor ion, such as erbium (Er), which acts as the laser-active ion.
The interest in Er-doped fiber amplifiers (EDFAs) has been due to their operating wavelength of 1.55 .mu.m that falls within the low absorption window of telecommunications fibers. They are efficient, i.e., a moderate amount of pump is wasted. EDFAs have gain over a wide spectral range that supports many high-speed telecommunications channels and can achieve gain of up to about 35 to 40 dB. The noise output from EDFAs is relatively low. They have long upper carrier lifetimes that minimize cross-talk between different communication channels. They are also small and compact.
Because of the advantages presented by EDFAs, interest in them has been particularly high in the area of telecommunications and, more lately, in laser satellite communications. But EDFAs have the disadvantage of a low pump absorption cross-section which requires very intense single transverse mode pump sources operating at 980 or 1450 nm.
Given the fact that a Yb-doped laser can operate at 980 nm, it would be advantageous to pump EDFAs with Yb-doped fiber lasers. And because of the closeness of pump and lasing wavelengths, relatively high slope efficiencies (output power vs. input power) can be achieved in Yb-doped fiber lasers.
Notwithstanding the desirability of Yb-doped fiber lasers operating at about 970 to 980 nm, the inherent three-level scheme makes such pumping problematic. At about 1030 nm and longer wavelengths, a Yb-doped fiber laser operates on the quasi four-level scheme. That energy level scheme requires only moderate inversion (less than 10%) to overcome the laser threshold and achieve gain. In contrast, the three-level scheme requires more than 50% of all Yb ions to be excited to the upper manifold to overcome the laser threshold. Thus, since the quasi four-level scheme requires less inversion than the three-level scheme, threshold conditions are often met first for the four-level scheme as power is pumped into the laser. And, there is a resulting tendency for the Yb ions to operate at 1030 nm or longer wavelengths.
Accordingly, unwanted or parasitic lasing at more than about 1020 nm may preclude achieving the desired lasing wavelength at about 980 nm. It can be appreciated that even small laser cavity reflections at parasitic wavelengths greater than about 1020 nm can result in a lower threshold for the parasitic wavelength than that for the desired one. As a result, lasing is first achieved at the longer wavelength, thereby clamping or holding the inversion at the threshold level for the parasitic wavelength. With the inversion clamped at a parasitic threshold, the desired inversion for 980 nm cannot be achieved. Further, even if the pump power is increased, the result is that inversion only occurs faster and more photons are omitted at longer wavelengths at the quasi four-level scheme and to the exclusion of the three-level scheme. Such competition of shorter vs. longer wavelengths occurs elsewhere over the Yb gain spectrum. For example, a desired Yb operation at 1064 nm may be hindered by parasitic lasing at 1090 nm, or 1030 nm lasing may be suppressed by 1060 nm lasing, etc. This is generally the case since higher inversions are required for a shorter vs. a longer wavelength.
As can be seen, there is a need for improved cladding pumped Yb-doped fiber lasers that can provide multi-Watt output. Additionally, there is a need for a Yb-doped fiber laser that operates at about 970-980 nm, which is the absorption bandwidth for EDFAs. Further, a fiber laser is needed to overcome parasitic lasing that occurs at wavelengths longer than the operating wavelength.